A common practice in the production of wire is the heat treatment thereof to effect patenting in which the lattice structure of the metal of the wire is converted by austenitizing it and then transforming it as completely as possible to a sorbite (granular martensite) lattice structure.
Since 1930, at least, efforts have been made to use electrical heating techniques to effect the heat treatment or patenting of wire so as to overcome the disadvantages of the classical methods whereby the wire was brought to patenting temperatures by radiation in appropriate heating chambers and over long residence times, especially when the wire was relatively thick. The electrical techniques were intended to utilize the resistance heating effect or the ohmic heat generated directly within the wire by the passage of the electric current therethrough.
The apparatus developed for this purpose generally comprises a preheating bath, usually of molten lead, in which the wire was heated to a temperature of 450.degree. C. to 550.degree. C. by drawing it continuously through the bath.
A similar bath served as a quenching bath and both baths were connected with a transformer so that the wire in the stretch between the baths was heated by the resistance heating or ohmic heat.
By appropriate selection of the transformer parameters and the length of the wire over the free stretch, i.e. the socalled annealing stretch between the point and the wire emerged from the first bath to the point at which the wire was immersed in the second bath, the wire could be brought in a relatively short time to the austenitizing temperatures of around 1,000.degree. C. which were required, this temperature being well above the Ac.sub.3 point. By passage then through the quenching bath, the wire was cooled to about 500.degree. C. and held at this temperature for about twenty seconds to allow the development of the desired fine grained sorbite (granular martensite) lattice structure.
While significant amounts of energy were required to heat the preheating bath, significant amounts of thermal energy must be dissipated from the quenching bath so that the overall energy consumption of the system was poor and the operation was highly expensive. In fact, the system required considerably more energy to operate the metal bath than was consumed in the heating of the metal wire between the metal baths.
Efforts have been made to reduce the energy consumption of such systems by, for example, heating the wire in at least one electrical induction furnace to the austenitizing temperature and then passing the wire through a single metal bath in which the sorbite lattice structure could be formed. In this case the metal bath served in part as a quenching bath but also as the preheating bath.
The advantage of this system is that it does bring about a reduction in the overall energy consumption, although difficulties have been encountered with it. For example, wire with a diameter of 3 mm or more cannot be effectively processed in the induction furnaces when a continuous production of the wire is required by providing the patenting system in the wire production line. In other words, with wires of a diameter above 3 mm, the heating of the wire in an induction furnace does not take place sufficiently rapidly to allow the system to be placed in line in the wire production plant.